The Liberal Democrats would be the ones who were squeezed - although they remain steady on 22 per cent of support if faced with Mr Blair, this would drop to 20 per cent if the contest were between Mr Cameron, Mr Brown and their leader, Menzies Campbell.

Today's poll is not good news for Mr Brown, who has recently tried to increase his appeal to voters by discussing issues beyond his normal Treasury brief.

He was boosted by the apparent endorsement of the prime minister during the Queen's speech debate last week, when Mr Blair warned the Tories would be facing a "heavyweight" at the next election. But this apparently is not enough for voters.

However, Labour has recovered from a record low in last month's ICM poll, which put the party at just 29 per cent of support. This suggests efforts to present a unified front after the infighting of September are making a difference with the electorate.

On Monday, Tory advertising guru and former party chairman Lord Saatchi warned that in moving to the centre ground, Mr Cameron risked alienating voters. But reports suggest he will ignore this advice and use a lecture on Friday to signal further changes.

In the Scarman lecture, the Tory leader is expected to call for a review of the party's welfare policies, arguing that the old idea of the welfare state being simply a safety net to keep people above the breadline is insufficient.